Iterating through an array, adding occurrences of a true in JavaScript

Suppose we have an array of true/false values represented by 't'/'f' which we retrieved from some database like this ?

const arr = ['f', 't', 'f', 't', 't', 't', 'f', 'f', 't', 't', 't', 't', 't', 't', 'f', 't'];
console.log(arr);

[ 'f', 't', 'f', 't', 't', 't', 'f', 'f', 't', 't', 't', 't', 't', 't', 'f', 't' ]

We need to count consecutive occurrences of 't' that are sandwiched between two 'f's and return an array of those counts.

Example

Here's how to count consecutive 't' values between 'f's:

const arr = ['f', 't', 'f', 't', 't', 't', 'f', 'f', 't', 't', 't', 't', 't', 't', 'f', 't'];

const countClusters = (arr = []) => {
    let res = [];
    res = arr.reduce((acc, val) => {
        const { length: l } = acc;
        if(val === 't'){
            acc[l - 1]++;
        }
        else if(acc[l - 1] !== 0){
            acc.push(0);
        }
        return acc;
    }, [0]);
    return res;
};

console.log(countClusters(arr));

[ 1, 3, 6, 1 ]

How It Works

The algorithm uses a reduce function that:

• Starts with an accumulator [0]
• When it encounters 't', it increments the last counter in the array
• When it encounters 'f' and the last counter is not zero, it starts a new counter by pushing 0
• This creates separate counts for each cluster of 't's between 'f's

Alternative Approach

Here's a more readable approach using a simple loop:

const arr = ['f', 't', 'f', 't', 't', 't', 'f', 'f', 't', 't', 't', 't', 't', 't', 'f', 't'];

const countClustersSimple = (arr) => {
    const result = [];
    let count = 0;
    
    for (let i = 0; i  0) {
            result.push(count);
            count = 0;
        }
    }
    
    return result;
};

console.log(countClustersSimple(arr));

[ 1, 3, 6, 1 ]

Conclusion

Both approaches effectively count consecutive 't' values between 'f's. The reduce method is more functional, while the loop approach is more straightforward and easier to understand.